buzz-cli `messages send` hard-fails the whole message (exit 1) when content contains a bare @-token that doesn't resolve to a channel member

### Bug
`buzz messages send --channel --content -` rejects the ENTIRE message whenever the content contains a bare (unquoted) `@`-token that does not match a current channel member, instead of sending the message as plain text.

The whole post is dropped and nothing is delivered.

### Repro
1. In a channel with members A and B, send:
`buzz messages send --channel --content "This mentions @-mention which is not a member"`
2. Result:
`{"error":"user_error","message":"mention '@-mention' does not match a current channel member; retry with --mention ","retryable":false}` (exit 1)
3. Control: wrapping the token so it's not a bare @-mention (e.g. quotes/backticks) sends fine.

### Expected
An unresolved-looking `@token` in ordinary content should not abort the whole send. Either send it as literal text, or drop just the token (mirroring how a display-name mention that can't be resolved is handled), rather than failing the message.

### Impact
Real/hard failure, silent for end users: any client or agent that emits prose containing a stray `@` (e.g. the literal word "@-mention") gets shut out — the message looks "sent" on the client side but never lands. This bit me repeatedly: an agent discussing the very concept of mentions kept writing `@-mention` in replies and every one of those replies vanished. Flapping/polling a reconnect does NOT help because the failure is content-based, not connection-based.

### Suggested layer (smallest first)
- [ ] Accept-and-ignore: if an @-token in content resolves to no member, send the rest as plain text (do not fail the message).
- [ ] Less invasive: only hard-fail when a *dedicated* `--mention` arg explicitly targets a non-member; treat bare @-tokens in content as literal text.
- [ ] Optional: emit a warning/event when a token was dropped so clients can log it.

Start by reproducing the issue with `buzz messages send --channel --content "This mentions @-mention"`, then trace the `messages send` path where bare @-tokens are resolved and rejected. Compare this with an explicit `--mention` targeting a non-member. Done means ordinary unresolved tokens no longer abort delivery, while the intended explicit-mention error behavior remains covered.

Written by the indexing model from the issue text.